This invention relates to a filament of the kind used in lamps, particularly, although not exclusively, in heater lamps. The invention is especially concerned with the support of such a filament within an envelope such as a quartz tube.
A popular form of heater lamp is the circular shaped tungsten halogen lamp used for cooker and heater applications. In such lamps, a filament, usually a coil of tungsten wire, is supported centrally within a quartz tube. An example of such a circular heater lamp is described in EP-A-0438254. As indicated in the drawings of the patent specification, the filament is supported at spaced intervals around the tube. A more detailed disclosure of examples of supports for a filament within a tube is given in EP-A-0020275, which relates to a linear filament lamp. Although the present invention will be described in connection with a heater lamp, it is equally applicable to incandescent lamps, i.e. lamps emitting visible light.
When a circular heating lamp, such as a cooking lamp, is at fill power the filament temperature is approximately 2300 K. If the heated filament is allowed to touch the quartz envelope, the quartz quickly degrades and leaks (due to de-vitrification and the formation of leak-paths along the grain boundaries), thereby rendering the lamp useless. The filament therefore needs to be securely maintained away from the wall of the envelope, and this may be done by providing circular filament supports, each comprising a wound tungsten ring that locks on to the filament and forms a spiral which centralises the filament within the quartz tube. Current designs of filaments use a coil having a pitch ratio within a certain range. The pitch ratio is defined as the pitch (between adjacent turns of the coil) divided by the diameter of the wire. This pitch ratio is typically between 1.2 and 1.9. For some applications, it is desirable to increase the pitch ratio above 1.9. However, once the pitch ratio is above about 1.8, the support no longer firmly grips, or locks on to, the filament, thereby enabling the filament to move within the support, and possibly to touch the envelope wall. This has the disadvantage that filament designs are constrained to the current range of pitch ratios, so that certain combinations of colour temperature, filament length and power rating are not possible.
It is an object of the present invention to overcome this disadvantage. According to the present invention there is provided an incandescent lamp filament comprising a helical coil of wire, characterised in that the coil comprises sections having at least two different pitches, including a first section having a pitch which enables the filament to be operated at a required colour temperature, and a second section having a pitch which enables the filament to be supported by support means which engages the coil.
The invention also provides a lamp comprising an envelope and a filament supported therein by a plurality of spaced supports, the filament being in accordance with the preceding paragraph, and having a plurality of said second sections spaced apart from one another and engaging respective ones of the supports.